The present invention relates to mobile communication equipment, more particularly to an antenna duplexer that can be used in the wireless interface of a device such as a mobile telephone.
The antenna duplexer of a mobile telephone or other mobile communication device must be small in size, and must combine low passband insertion loss with high stopband attenuation. As the density of mobile communications in the radio frequency spectrum increases, the antenna duplexer must also be able to deal with increasingly close spacing between transmit and receive channels, and there is a further trend toward increasing bandwidths of these channels.
Antenna duplexers employing surface-acoustic-wave (SAW) filters are particularly suitable for meeting these requirements. Examples of antenna duplexers of this type can be found in Japanese Unexamined Patent Applications 9-98046 and 5-167388. FIG. 7 of the latter document (5-167388) is reproduced here as FIG. 1.
The antenna duplexer shown in FIG. 1 has two SAW filters F1, F2 coupled in parallel via nodes xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99 to a pair of common signal terminals T0. SAW filter F1 leads to another pair of signal terminals T1; SAW filter F2 leads to still another pair of signal terminals T2. Each SAW filter F1, F2 has a ladder network configuration comprising a series resonator (denoted RS0 in F1 and RS in F2) and a shunt resonator (denoted RP in F1 and RP0 in F2). Each of these resonators has a single pair of interdigital transducers (not visible) flanked by a pair of reflectors (not visible). The interdigital transducers and reflectors are formed from, for example, an aluminum-copper (Al-2% Cu) electrode material disposed on a lithium-tantalate (LT) substrate. As seen from the common signal terminals T0, series resonator RS0 forms the first stage of filter F1, and shunt resonator RP0 forms the first stage of filter F2. If necessary, each filter F1, F2 may have additional series resonators and shunt resonators following one another in the same ladder pattern.
Filters F1 and F2 are bandpass filters with respective center frequencies f1 and f2, where f1 is less than f2. The passbands of the two filters F1, F2 must not overlap. Filter F2 must have a high characteristic impedance in the passband of filter F1, and filter F1 must have a high characteristic impedance in the passband of filter F2. In the antenna duplexer in FIG. 26, an extra length of signal line S is inserted in series with filter F2 to increase its lower stopband impedance by producing a phase rotation.
An f1 value of eight hundred eighty-seven megahertz (887 MHz) and an f2 value of nine hundred thirty-two megahertz (932 MHz) are given as examples in the above-mentioned document (5-167388); these are the transmit frequency (f1) and receive frequency (f2) of a type of cordless telephone. When the antenna duplexer is used in this type of cordless telephone, the common terminals T0 are coupled to the antenna, terminals T1 are coupled to a transmitting circuit, and terminals T2 are coupled to a receiving circuit. A cordless telephone has comparatively narrow transmit and receive bands, so the requirement that each filter must have a high stopband impedance in the other filter""s passband is readily met by the simple circuit configuration in FIG. 1.
When this antenna duplexer configuration is used in a mobile communication system such as the Japanese CDMA-One system, in which the transmit frequency is higher than the receive frequency, filter F1 becomes the receive filter and filter F2 becomes the transmit filter. The Japanese CDMA-One system, however, has comparatively wide transmit-band and receive-band specifications; the proportional bandwidth (the bandwidth divided by the center frequency) is equal to 0.042 for the transmit band and 0.045 for the receive band. Moreover, the spacing between the lowest transmit frequency and the highest receive frequency is less than half the passband width. The simple duplexer configuration in FIG. 1 fails to provide the desired combination of low passband insertion loss and high stopband impedance for this system, and for other mobile communication systems with wide, closely spaced transmit and receive bands.
An object of the present invention is to provide an antenna duplexer that combines low insertion loss in both the transmit and receive passbands with an adequately high characteristic impedance in the relevant stopbands (the transmit stopband in which the receive passband is located, and the receive stopband in which the transmit passband is located).
The invented antenna duplexer has a common node, first and second signal terminals, a first filter coupled between the common node and the first signal terminal, a second filter coupled between the common node and the second signal terminal, and an antenna signal terminal coupled to the common node. The first and second filters each have at least one series resonator and at least one shunt resonator coupled in a ladder configuration. The first filter has a higher passband than the second filter.
The antenna duplexer also has a transmission-line circuit with an internal node, a first transmission line coupling the internal node to the common node, a second transmission line coupling the internal node to the first filter, and a grounding circuit coupling the internal node to ground. The grounding circuit comprises, for example a capacitor coupled between the internal node and a ground node.
The grounding circuit increases the stopband impedance of the first filter in the region of the passband of the second filter, and improves impedance matching as seen from the antenna signal terminal.